The objective is to characterize the biochemical and genetic mechanisms of obligate intracellular parasitism between the eukaryotic host and prokaryotic parasite. Our recent studies have suggested that a key feature of intraphagolysosomal growth of Coxiella burnetii, the etiological agent of Q fever, is a membrane structurally and functionally compatible with the acidic and otherwide hostile environment of the phagolysosome. Thus, we have proposed a pathogenic mechanism for C. burnetti cased on regulation of microbial metabolism by H1 - dependent stimulation of cell function. We have continued to probe the molecular mechanisms of this pH dependency and have demonstrated that C. burnetii behaves similarly to other acidophilic bacteia in that it maintains an intracellular pH more alkaline than the environment and that this pH gradient is used to energize glutamate transport. The maintenance of the ph gradient is at least partially energy dependent. The stability of C. burnetii in the environment may be explained, in part, by the lack of a requirement for metabolic energy to maintain acceptable intracellular pH values in neutral or alkaline environments.